Phase shifter mask

ABSTRACT

A phase shift mask (PSM) includes a light transmitting substrate. The PSM further includes a phase shifter over the light transmitting substrate, wherein the phase shifter includes a plurality of semiconductor layers and a plurality of dielectric layers stacked in an alternating fashion.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No.15/399,205, filed Jan. 5, 2017, which claims the priority of U.S.Provisional Application No. 62/427,643, filed Nov. 29, 2016, which areincorporated herein by reference in their entireties.

BACKGROUND

Photolithography is utilized in the fabrication of semiconductor devicesto transfer a pattern onto a wafer. Based on various integrated circuit(IC) layouts, patterns are transferred from a photomask to a surface ofthe wafer. The photomasks, also called reticles, are made of quartz orglass with one or more metallic materials deposited on one side toprevent light penetration. As dimensions decrease and density in ICchips increases, resolution enhancement techniques, such as opticalproximity correction (OPC), off-axis illumination (OAI), double dipolelithography (DDL) and phase-shift mask (PSM), are developed to improvedepth of focus (DOF) and therefore to achieve a better pattern transferonto the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a flow chart of a method of fabricating a photomask inaccordance with one or more embodiments.

FIG. 1B is a flow chart of a method of forming a phase shifter inaccordance with one or more embodiments.

FIGS. 2A-2E are cross-sectional views of a photomask at various stagesof production in accordance with one or more embodiments.

FIG. 3 is a cross-sectional view of a photomask in accordance with oneor more embodiments.

FIG. 4 is a cross-sectional view of a photomask in accordance with oneor more embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components, values, operations, materials,arrangements, or the like, are described below to simplify the presentdisclosure. These are, of course, merely examples and are not intendedto be limiting. Other components, values, operations, materials,arrangements, or the like, are contemplated. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As semiconductor device feature sizes have decreased to be smaller thana wavelength of light used in photolithography processes, the ability ofmanufacturing the minimum feature size, also called critical dimensions(CD), become sensitive to optical fringing of light passing through aphotomask or a reticle. Because of constructive and destructiveinterference effects, also referred to as diffraction, photoresist at anedge of a defined pattern is exposed under undesired light, resulting ina distortion in a pattern transferred to a wafer. In order to enhance aresolution during the image transfer, a phase shift mask (PSM) is usedto shift a phase of selected light passing through the photomask or thereticle by π (180 degrees), thereby the undesired light is scattered oroffset by the destructive interference. Removing the undesired lighthelps to improve the precision of the image transfer. Typically, the PSMis categorized into an alternating PSM or an attenuated PSM. Thealternating PSM induces the phase shift of light by adjusting athickness of a clear region. The attenuated PSM allows a smallpercentage of light to penetrate through a dark region. In someinstances, each PSM includes molybdenum silicide (MoSi) as a phaseshifter. Defects such as crystal haze and particles are generated aftera series process including UV exposure, baking and cleaning by sulfuricacid and ammonia, thereby causing a greater CD error and decreasingmanufacturing yield.

In some embodiments, in order to reduce a growth of crystal haze, acombination of a semiconductor layer, such as silicon, and a dielectriclayer, such as silicon dioxide, is used in place of MoSi. In someembodiments, the phase shifter includes from 2 to 12semiconductor/dielectric layer pairs. Based on a control of etchselectivities, such a phase shifter has an improved profile during anetch process and a reduced CD loss during the photolithography process.In some embodiments, a bottom layer of the phase shifter has arelatively lower etch rate during the etch process. As a result, anunetched protrusion is formed between the phase shifter and atransparent substrate. The unetched protrusion enhances a physicaldamage resistance of the phase shifter to a subsequent clean process.

FIG. 1A is a flow chart of a method 100A of fabricating a photomask inaccordance with one or more embodiments. One of ordinary skill in theart would understand that additional operations are able to be performedbefore, during, and/or after method 100A depicted in FIG. 1A, in someembodiments. Method 100A includes operation 110 in which a phase shifteris deposited over a light transmitting substrate. A selected singlewavelength or waveband is intended to penetrate through the lighttransmitting substrate. In some embodiments, the light transmittingsubstrate is deemed transparent under near ultra violet (NUV)wavelengths (e.g., less than 365 nanometers (nm)). In some embodiments,the light transmitting substrate is deemed transparent under deep ultraviolet (DUV) wavelengths (e.g., less than 284 nm). In some embodiments,the light transmitting substrate is deemed transparent under argonfluoride (ArF) laser (e.g., 193 nm). The phase shifter, also referred toas a semi-light transparent phase shifter, is used to change a phase oflight transmitted by the light transmitting substrate. The phase shifterincludes a plurality of semiconductor layers and a plurality ofdielectric layers arranged in an alternating fashion. The semiconductorlayer and the dielectric layer have different etch selectivities. Insome embodiments, the formation of the phase shifter includes adeposition process and an etch process performed cyclically.

FIG. 1B is a flow chart of a method 100B of forming a phase shifter inaccordance with one or more embodiments. One of ordinary skill in theart would understand that additional operations are able to be performedbefore, during, and/or after method 100B depicted in FIG. 1B, in someembodiments. Method 100B includes operation 112 in which a firstsemiconductor layer is deposited over the light transmitting substrate.The first semiconductor layer is used to endure an ion bombardment andtherefore prevent a bottom portion of the phase shifter from being overetched. In some embodiments, the first semiconductor layer includessilicon, germanium, silicon germanium, silicon carbide or anothersuitable material. In some embodiments, the formation of the firstsemiconductor includes a deposition process, such as a chemical vapordeposition (CVD).

In operation 114, at least one first dielectric layer and at least onesecond semiconductor layer are deposited, in an alternating fashion,over the first semiconductor layer. When more than one secondsemiconductor layer is deposited, in some embodiments, each of thesecond semiconductor layers includes the same material. In someembodiments, at least one second semiconductor layer is different fromthe first semiconductor layer or another second semiconductor layer. Insome embodiments, each of the at least one second semiconductor layerincludes the same material as the first semiconductor layer. Forexample, the first semiconductor layer includes silicon and the at leastone second semiconductor layer includes germanium. In some embodiments,the formation process of the second semiconductor layer includesplasma-enhanced CVD (PECVD), high-density plasma CVD (HDPCVD), lowpressure CVD (LPCVD) or another suitable process.

In some embodiments, the at least one first dielectric layer includes asingle dielectric layer. In some embodiments, the at least one firstdielectric layer includes multiple dielectric layers adjacent oneanother, for example, a silicon nitride layer and a silicon oxide layerwithout an intervening semiconductor layer. In some embodiments, theformation of the at least one first dielectric layer includes the samedeposition process as that used to form the second semiconductor layer.In some embodiments, the formation of the at least one first dielectriclayer includes a different deposition process from that used to form thesecond semiconductor layer. For example, the first dielectric layer usesa CVD process and the second semiconductor layer uses an atomic layerdeposition (ALD) process. In some embodiments, each of the at least onefirst dielectric layer is formed by the same process. In someembodiments, at least one of the first dielectric layer is formed by adifferent process from another. In some embodiments, each of the atleast one first dielectric layer has a same material. In someembodiments, at least one of the at least one first dielectric layer hasa different material from another of the at least one first dielectriclayer. In some embodiments, each of the at least one first dielectriclayer is a single dielectric layer. In some embodiments, each of the atleast one first dielectric layer includes multiple dielectric layersadjacent one another. In some embodiments, at least one of the at leastone first dielectric layer is a single dielectric layer and another ofthe at least one first dielectric layer includes multiple dielectriclayers adjacent one another.

In operation 116, a second dielectric layer is deposited over the atleast one first dielectric layer and the at least one secondsemiconductor layer. In some embodiments, the second dielectric layerincludes the same material as each of the at least one first dielectriclayer. In some embodiments, the second dielectric layer includes adifferent material from at least one of the at least one the firstdielectric layer. In some embodiments, the formation of the seconddielectric layer is the same as that used to form the at least one firstdielectric layer. In some embodiments, the formation of the seconddielectric layer is different from that used to form the at least onefirst dielectric marital. For example, second dielectric layer uses aCVD process and the at least one first dielectric uses an ALD process.Based on the photolithographic parameters, a thickness and a refractionindex of the phase shifter is determined by selecting the material andformation of the first semiconductor layer, the at least one secondsemiconductor layer, the at least one first dielectric layer and thesecond dielectric layer.

FIG. 2A is a cross-sectional view of a photomask 200 following operation110 in accordance with one or more embodiments. Photomask 200 includes alight transmitting substrate 210 and a phase shifter 220. In someembodiments, light transmitting substrate 210 is formed of glass, fusedsilica, quartz, calcium fluoride, sapphire or another suitable material.In some embodiments, a thickness of light transmitting substrate 210ranges from about 0.09 inches for a five-inch mask to about 0.25 inchesfor a six-inch mask If the thickness is too small, photomask 200 will befragile and a risk of cracking or breaking during handling photomask 200increases, in some instances. If the thickness is too great, a cost oflight transmitting substrate 210 will increase without a significantincrease in functionality, in some instances.

Phase shifter 220 includes a first semiconductor layer 222, a firstdielectric layer 224, a second semiconductor layer 226 and a seconddielectric layer 228. In some embodiments, photomask 200 includes one ormore semiconductor layers or dielectric layers stacked between secondsemiconductor layer 226 and second dielectric layer 228 in analternating fashion. In some embodiments, including layers 222-228,phase shifter 220 has from 2 to 12 semiconductor/dielectric pairs,collectively referred to as pairs P. If a quantity of pairs P is toosmall, a sidewall of a subsequently etched phase shifter 220 will have anotch or have a tapered shape, in some instances. If a quantity of pairsP is too great, a cost of manufacturing phase shifter 220 will increasewithout a significant increase in functionality, in some instances.

In some embodiments, first semiconductor layer 222, second semiconductorlayer 226 and other semiconductor layers within phase shifter 220,collectively referred to as semiconductor layers S, independentlyinclude silicon, germanium, silicon germanium, silicon carbide oranother suitable material. In some embodiments, first dielectric layer224, second dielectric layer 228 and other dielectric layers withinphase shifter 220, collectively referred to as dielectric layers D,independently include silicon dioxide, silicon nitride, siliconoxynitride or another suitable material. In some embodiments, each layerof dielectric layer D includes a single layer. In some embodiments, eachlayer of dielectric layers D includes multiple layers, for example, acombination of silicon dioxide and silicon nitride. The quantity ofpairs P and the material selected for semiconductor layers S and fordielectric layers D are adjustable based on various parameters requiredin the photolithography process, such as transmittance, optical density,refractive index or critical dimension (CD) loss. In at least oneembodiment where each layer of semiconductor layers S is silicon, eachlayer of dielectric layer D includes silicon dioxide or silicon nitride.The formation of phase shifter 220 includes a deposition process, suchas ALD or CVD.

In some embodiments, a thickness of each semiconductor layer Sindependently ranges from about 1 nm to about 5 nm. If the thickness istoo small, phase shifter 220 will suffer more damage during thesubsequent clean or etch process, in some instances. If the thickness istoo great, a transmittance of phase shifter 220 will decrease, in someinstances. In some embodiments, each of semiconductor layer S has thesame thickness. In some embodiments, at least one of semiconductor layerS has a different thickness from another semiconductor layer. In someembodiments, a thickness of each dielectric layer D independently rangesfrom about 10 nm to about 20 nm. If the thickness of a dielectric layeris too great, a sidewall of the subsequently etched phase shifter 220will be irregular, in some instances. If the thickness of a dielectriclayer is too small, an optical property of phase shifter 220 will behard to control, in some instances. In some embodiments, each ofdielectric layer D has the same thickness. In some embodiments, at leastone of dielectric layer D has a different thickness from anotherdielectric layer. In some embodiments, when photomask 200 is anattenuated PSM, a total transmission rate incident light of phaseshifter 220 ranges of from about 6% to 18%, in some instances. If thetransmission rate is too great or too small, an intensity amplitude ofphase-shifted light will be too much or insufficient, so the resolutionenhancement of the image to be transferred will decrease, in someinstances. Based on the inherent physical property and etching method,semiconductor layers S have a lower etch rate than dielectric layer D.For example, during a dry etch process, because silicon endures greaterion bombardments than silicon dioxide or silicon nitride, silicon has alower etch rate than silicon dioxide or silicon nitride. Therefore, thesidewall profile is improved because of a multilayer structure with acombination of silicon and silicon dioxide/silicon nitride. In someembodiments, a ratio of etch rates between dielectric layer D andsemiconductor layer S ranges from about 1.5 to about 2.5. If the ratiois too great, a loss of dielectric layer D will increase resulting indistortion of the image to be transferred, in some instances. If theratio is too small, a manufacturing cost of photomask 200 will increase,in some instances.

Returning to FIG. 1A, method 100A continues with operation 120 in whicha shading layer is deposited over the phase shifter. The shading layeracts as a light absorber in the photomask. In some embodiments, theshading layer includes a metal material. In some embodiments, theshading layer includes a metal material and an oxide material. In someembodiments, the shading layer includes a metal material and a metaloxide gradient. In some embodiments, the metal oxide gradient or a metalis used to reduce reflectivity during the photolithography process.

FIG. 2B is a cross-sectional view of photomask 200 following operation120 in accordance with one or more embodiments. A shading layer 230,also referred to as an opaque layer, is over phase shifter 220. In someembodiments, shading layer 230 includes chromium, chromium oxide,chromium oxynitride or another suitable material. The formation ofshading layer 230 includes a deposition process, such as sputtering,CVD, physical vapor deposition (PVD), ALD or another suitable process. Athickness of shading layer 230 is based on various designs ofphotomasks. In some embodiments, when photomask 200 is the attenuatedPSM, the thickness of shading layer 230 ranges from about 40 nm to about65 nm, in some instances. If the thickness is too great, an intensity ofphase-shifted light penetrating through shading layer 230 decreases, insome instances. If the thickness is too small, a side lobe of thephotoresist will be affected by phase-shifted light resulting indistortion of the image to be transferred, in some instances. In someembodiments, when photomask 200 is an alternating PSM, the thickness ofshading layer 230 is greater than 100 nm, in some instances. If thethickness is too small, an amount of light absorption will beinsufficient, in some instances. Similarly, an ability of light phaseshifter 220 and/or shading layer 230 to absorb or block the passage oflight, also referred to as an optical density, is based on variousdesigns of photomasks. For example, for the alternating PSM, the opticaldensity of a combination of shading layer 230 and phase shifter 220 isgreater than 3. If the optical density is too small, the light blockagewill be insufficient, in some instances.

Returning to FIG. 1A, method 100A continues with operation 130 in whicha first portion of the shading layer and a portion of the phase shifterare removed to expose a portion of the light transmitting substrate. Insome embodiments, a first mask layer is formed over the shading layer todefine a first pattern. Next, a first portion of the shading layer and aportion of the phase shifter thereunder are removed to expose a portionof the light transmitting substrate. Such formation includes aphotolithography and an etch process. In some embodiments, the removalof the first portion of the shading layer and the portion of the phaseshifter thereunder uses laser-beam writing to expose the portion of thelight transmitting substrate.

FIG. 2C is a cross-sectional view of photomask 200 following operation130 in accordance with one or more embodiments. In some embodiments, afirst photoresist is formed and patterned over shading layer 230,followed by an etch process to remove a first portion of shading layer230. Next, shading layer 230 is used as a hard mask for a removal of aportion of phase shifter 220. As a result, a portion of lighttransmitting substrate 210 is exposed. Alternatively, the portion ofshading layer 230 and the portion of phase shifter 220 are removed inthe same etch process. In some embodiments, a region 240 of an exposedlight transmitting substrate 210 s is referred to as a clear tone and aregion 242 of phase shifter 220 and shading layer 230 is referred to asa dark tone. In some embodiments, after operation 130, photomask 200 isreferred to as the attenuated PSM. In some embodiments, the etch processincludes a dry etching, such as a plasma etching, a wet etching or acombination of the dry etching and the wet etching. In some embodimentwhere the etch process is a plasma etching, an etchant gas includes afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆),chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃),bromine-containing gas (e.g., HBr and/or CHBR₃), or another suitablegases. For example, when the etchant gas is a mixture of SF₆ and oxygen,silicon has a relatively lower etch rate than silicon nitride andsilicon dioxide. Because semiconductor layer S has a lower etch ratethan dielectric layer D, semiconductor layer S acts as anion-bombardment barrier during the etch process, in some instances.Therefore, photomask 200 has an improved sidewall profile relative to aconventional MoSi phase shifter, which helps to enhance the resolutionand reduce CD error during the semiconductor manufacturing. In someembodiments, the removal process uses laser-beam writing.

In some embodiments, under an exposure energy of around 48,000 Joules,compared to layout designed patterns, an undesired CD growth ofphotomask 200 is smaller than 0.1 nm and an undesired CD loss ofphotomask 200 is smaller than 0.1 nm. In some embodiments, under anexposure energy of around 8,000 Joules, each photomask 200 is able toproduce at least 25,000 wafers.

FIG. 2D is a schematic cross-sectional view of photomask 200 inaccordance with one or more embodiments. After the removal process, aportion of unetched first semiconductor layer 222, also referred to as aprotrusion 222′, extends from a sidewall of first semiconductor layer222 over light transmitting substrate 210. In some embodiments,protrusion 222′ has a tapered profile. In some embodiments whereprotrusion 222′ has a tapered profile, a thickness and a width ofprotrusion 222′ ranges from about 0.1 nm to about 1 nm. If the thicknessor the width is too great, a contrast between clear tone region 240 anddark tone region 242 will decrease, in some instances. If the thicknessor the width is too small, a physical resistance during a subsequentclean or etch process will decrease, increasing a risk of collapse ofphase shifter 220, in some embodiments. In some embodiments, each ofsemiconductor layer S forms a protrusion after the ion bombardmentprocess. Similar to protrusion 222′, each protrusion of semiconductorlayer S has a thickness ranging from about 0.1 nm to about 1 nm and awidth ranging from about 0.1 nm to about 1 nm.

Returning to FIG. 1A, method 100A continues with an optional operation140 in which a portion of the light transmitting substrate is removed.In some embodiments, a second mask layer is formed over the shadinglayer and the exposed portion of the light transmitting substrate todefine a second pattern. Next, a portion of the light transmittingsubstrate is removed to form a recess. Such formation includes aphotolithography and an etch process.

FIG. 2E is a cross-sectional view of photomask 200 following operation140 in accordance with one or more embodiments. A second photoresist isformed and patterned over shading layer 230 and exposed lighttransmitting substrate 210 s, followed by an etch process to remove aportion of light transmitting substrate 210. As a result, a recess 250is formed in light transmitting substrate 210. In some embodiments, theremoval of light transmitting substrate 210 uses the same process as theremoval of shading layer 230 or phase shifter 220. In some embodiments,the removal of light transmitting substrate 210 uses a different processfrom the removal of shading layer 230 and phase shifter 220. A depth ofrecess 250 is selected based on a refraction index of light transmittingsubstrate and wavelength of incident light to realize a phase shift. Insome embodiment, the depth of recess 250 ranges from about 1 nm to about4 nm. A greater or smaller depth negatively affects a resolution ofphotomask 200, in some instances. In some embodiments, when photomask200 is a tri-tone PSM, recess 250 is referred to as a phase shiftingtone 244, exposed portion of light transmitting substrate 210 s isreferred to a clear tone, and a region of stacked phase shifter 220 andshading layer 230 is referred to as a dark tone 246. In someembodiments, according to different manufacturing requirements, shadinglayer 230 is partially or completely removed. In some embodiments,protrusion 222′ (best seen in FIG. 2D) is at a sidewall of recess 250.Protrusion 222′ has a sufficiently small roughness to avoid impactingthe resolution of photomask 200.

FIG. 3 is a cross-sectional view of a photomask 300 in accordance withone or more embodiments. Photomask 300 is similar to photomask 200, likeelements have a same reference number increased by 100. In contrast withphotomask 200, a shading layer 330 is between a light transmittingsubstrate 310 and a phase shifter 320. In some embodiments, theformation of shading layer 330 and phase shifter 320 is the same as theformation of shading layer 230 and phase shifter 220. Photomask 300 isformed by adjusting an order of operations in method 100A, in someembodiments. For example, operation 120 is performed prior to operation110.

FIG. 4 is a cross-sectional view of a photomask 400 in accordance withone or more embodiments. Photomask 400 is similar to photomask 200, likeelements have a same reference number increased by 200. In contrast withphotomask 200 (best see in FIG. 2E), a shading layer 430 is between alight transmitting substrate 410 and a phase shifter 420. Similar tophotomask 200, a recess 450 is formed in light transmitting substrate410 to form a tri-tone PSM, in some instances. In some embodiments, theformation of recess 450 is the same as the formation of recess 250. Insome embodiments, recess 450 is referred to a phase shifting tone region444, unetched light transmitting substrate 410 s is referred to a cleartone region 440, and a region of stacked phase shifter 420 and shadinglayer 430 is referred to as a dark tone region 446.

One of ordinary skill in the art would understand that photomasks200-400 will undergo further processing to complete fabrication. Forexample, in at least one embodiment, a third mask layer is formed overthe photomask to define a third pattern. As another example, apassivation layer is optionally deposited over photomasks 200-400 afteroperation 130 or 140 or (depending on various designs of photomasks) torepair defects generated during the manufacturing process.

The insertion of semiconductor layers with a relatively lower etch ratethan dielectric layers helps keep a sidewall profile of phase shifter,resulting in an improved CD pattern during the photolithography process.In addition, a transmittance of the phase shifter is adjustable byvarious combinations of the semiconductor layer and dielectric layer.Further, comparing to molybdenum silicide-based material, thesemiconductor layer, such as silicon, and the dielectric layer, such assilicon dioxide, help reduce CD increase caused by oxidation and reducea risk of haze caused during clean process. Moreover, an un-etchedprotrusion of the bottom semiconductor layer enhances a damageresistance caused by a wet clean process, resulting in a reducedmanufacturing cost and production yield.

An aspect of this description relates to a phase shift mask (PSM). ThePSM includes a light transmitting substrate. The PSM further includes aphase shifter over the light transmitting substrate, wherein the phaseshifter includes a plurality of semiconductor layers and a plurality ofdielectric layers stacked in an alternating fashion. In someembodiments, the PSM further includes a first opening through the phaseshifter; and a second opening through the phase shifter. In someembodiments, a depth of the first opening is equal to a depth of thesecond opening. In some embodiments, a depth of the first opening isdifferent from a depth of the second opening. In some embodiments, asidewall of the phase shifter in the first opening is tapered. In someembodiments, the PSM further includes a shading layer over the lighttransmitting substrate. In some embodiments, the shading layer isbetween the light transmitting substrate and the phase shifter. In someembodiments, the phase shifter is between the light transmittingsubstrate and the shading layer.

An aspect of this description relates to a phase shift mask (PSM). ThePSM includes a light transmitting substrate. The PSM further includes aphase shifter over the light transmitting substrate, wherein the phaseshifter comprising a first semiconductor layer, at least one pair of afirst dielectric layer and a second semiconductor layer over the firstsemiconductor layer, and a second dielectric layer over the at least onepair of the first dielectric layer and the second semiconductor layer.The PSM further includes a shading layer over the phase shifter. In someembodiments, the phase shifter and the shading layer define a pluralityof openings. In some embodiments, each opening of the plurality ofopenings has a same depth. In some embodiments, at least one opening ofthe plurality of openings has a different depth from another opening ofthe plurality of openings. In some embodiments, a sidewall of the phaseshifter defining a first opening of the plurality of openings istapered. In some embodiments, a sidewall of the phase shifter defining afirst opening of the plurality of openings comprises a curved portion.In some embodiments, the PSM is a tri-tone PSM.

An aspect of this description relates to a phase shift mask (PSM). ThePSM includes a light transmitting substrate. The PSM further includes aplurality of phase shifter stacks over the light transmitting substrate,wherein each of the plurality of phase shifter stacks comprises aplurality of semiconductor layers and a plurality of dielectric layersstacked in an alternating fashion. The PSM further includes a pluralityof openings, wherein adjacent phase shifter stacks of the plurality ofphase shifter stacks define a corresponding opening of the plurality ofopenings. In some embodiments, a top surface of the light transmittingsubstrate exposed by a first opening of the plurality of openings isco-planar with a top surface of the light transmitting substrate exposedby a second opening of the plurality of openings. In some embodiments, atop surface of the light transmitting substrate exposed by a firstopening of the plurality of openings is offset in a directionperpendicular to the top surface with respect to a top surface of thelight transmitting substrate exposed by a second opening of theplurality of openings. In some embodiments, the PSM further includes aplurality of shading layers, wherein each shading layer of the pluralityof shading layers is between the light transmitting substrate and acorresponding phase shifter stack of the plurality of phase shifterstacks. In some embodiments, the PSM further includes a plurality ofshading layers, wherein each shading layer of the plurality of shadinglayers is separated from the light transmitting substrate by acorresponding phase shifter stack of the plurality of phase shifterstacks.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A phase shift mask (PSM), comprising: a lighttransmitting substrate; and a phase shifter over the light transmittingsubstrate, wherein the phase shifter comprises a plurality ofsemiconductor layers and a plurality of dielectric layers stacked in analternating fashion.
 2. The PSM of claim 1, further comprising: a firstopening through the phase shifter; and a second opening through thephase shifter.
 3. The PSM of claim 2, wherein a depth of the firstopening is equal to a depth of the second opening.
 4. The PSM of claim2, wherein a depth of the first opening is different from a depth of thesecond opening.
 5. The PSM of claim 2, wherein a sidewall of the phaseshifter in the first opening is tapered.
 6. The PSM of claim 1, furthercomprising a shading layer over the light transmitting substrate.
 7. ThePSM of claim 6, wherein the shading layer is between the lighttransmitting substrate and the phase shifter.
 8. The PSM of claim 6,wherein the phase shifter is between the light transmitting substrateand the shading layer.
 9. A phase shift mask (PSM), comprising: a lighttransmitting substrate; a phase shifter over the light transmittingsubstrate, wherein the phase shifter comprising a first semiconductorlayer, at least one pair of a first dielectric layer and a secondsemiconductor layer over the first semiconductor layer, and a seconddielectric layer over the at least one pair of the first dielectriclayer and the second semiconductor layer; and a shading layer over thephase shifter.
 10. The PSM of claim 9, wherein the phase shifter and theshading layer define a plurality of openings.
 11. The PSM of claim 10,wherein each opening of the plurality of openings has a same depth. 12.The PSM of claim 10, wherein at least one opening of the plurality ofopenings has a different depth from another opening of the plurality ofopenings.
 13. The PSM of claim 10, wherein a sidewall of the phaseshifter defining a first opening of the plurality of openings istapered.
 14. The PSM of claim 10, wherein a sidewall of the phaseshifter defining a first opening of the plurality of openings comprisesa curved portion.
 15. The PSM of claim 9, wherein the PSM is a tri-tonePSM.
 16. A phase shift mask (PSM), comprising: a light transmittingsubstrate; a plurality of phase shifter stacks over the lighttransmitting substrate, wherein each of the plurality of phase shifterstacks comprises a plurality of semiconductor layers and a plurality ofdielectric layers stacked in an alternating fashion; and a plurality ofopenings, wherein adjacent phase shifter stacks of the plurality ofphase shifter stacks define a corresponding opening of the plurality ofopenings.
 17. The PSM of claim 16, wherein a top surface of the lighttransmitting substrate exposed by a first opening of the plurality ofopenings is co-planar with a top surface of the light transmittingsubstrate exposed by a second opening of the plurality of openings. 18.The PSM of claim 16, wherein a top surface of the light transmittingsubstrate exposed by a first opening of the plurality of openings isoffset in a direction perpendicular to the top surface with respect to atop surface of the light transmitting substrate exposed by a secondopening of the plurality of openings.
 19. The PSM of claim 16, furthercomprising a plurality of shading layers, wherein each shading layer ofthe plurality of shading layers is between the light transmittingsubstrate and a corresponding phase shifter stack of the plurality ofphase shifter stacks.
 20. The PSM of claim 16, further comprising aplurality of shading layers, wherein each shading layer of the pluralityof shading layers is separated from the light transmitting substrate bya corresponding phase shifter stack of the plurality of phase shifterstacks.